The Habitable Worlds Observatory (HWO) is a future NASA flagship mission which will use a segmented telescope and coronagraphic instruments to discover and characterize exoplanets, including exoEarths – Earth-like planets orbiting other stars. HWO will require extraordinary optical stability, with wavefront drift performance measured in the picometers. This paper explores how active control of the telescope optics, using metrology systems that include laser distance gauges, segment edge sensors, and picometer precision actuators, can provide the needed telescope stability. Together with wavefront sensing and deformable mirrors in the coronagraph, this approach can control the entire coronagraphic beam train, to stabilize the electric field in the coronagraph. The HWO Technology Assessment Group is developing three “Exploratory Analytic Cases,” which are conceptual designs for HWO that differ in some respects, to provide a basis for detailed analysis. This paper addresses EAC1, a deployed-aperture concept that draws on JWST heritage. EAC1 uses 19 1.8-meter hexagonal segments to form its off-axis Primary Mirror (PM), as sketched in Figure 1. EAC2 will use fewer, larger “keystone” segments in a non-deployed off-axis PM configuration, and EAC3 will be a larger, on-axis deployed telescope using smaller keystone shaped segments.
The Habitable Worlds Observatory will have uniquely stringent wavefront stability requirements, in the single-digit picometers for observations lasting days, to preserve coronagraph contrast for imaging earth-like exoplanets. This need will be addressed using high-precision Wavefront Sensing and Control methods, including continuous picometerprecision metrology and control of the Optical Telescope Assembly (OTA). This paper reviews methods for initializing and maintaining the OTA wavefront, evolved from those used for the James Webb Space Telescope, but extended to much higher precision. It concludes by identifying performance targets for WFSC technology development, to help guide NASA technology investments.
We propose an approach for coarse alignment of a segmented space telescope using science instrument images. The recommended steps go from large post launch rigid body misalignments to within the capture range of coarse phasing where segment piston error is the predominant residual wavefront error. These steps include five data collection and analysis methods comprising of metrology capture, segment capture and identification, segment translation, segment stacking, and fine alignment. Using a proposed architecture for the NASA Habitable Worlds Observatory (HWO) we describe the details of our recommended approach for each telescope alignment step. We then compare this recommended sequence to alternative alignment progressions used in existing segmented testbeds and telescopes in terms of number of data collections required. This model-based demonstration establishes that the recommended coarse and fine alignment sequence performs more efficiently in time and resource cost, handing off to coarse and fine phasing activities further along the telescope commissioning process.
Current approaches for phasing of segmented space telescopes have required complex dedicated optics and mechanisms, such as Dispersed Hartmann sensors or grisms. These methods do not scale well as the number of segments increases. The broadband phasing approach used at the Keck Observatory does scale well and can work on space telescopes without the need for any additional hardware. We show that this method implemented as white light interferometry (WLI), using a standard imaging detector and filters, has a capture range limited only by the range of the segment actuators and can easily phase the mirrors to within the capture range of single wavelength phasing methods. An analysis of the Keck broadband phasing performance is presented and used to develop a formula for implementation of WLI on other segmented telescopes. As an example, a WLI implementation for the NASA Habitable Worlds Observatory telescope is developed and demonstrated via detailed wave-optics simulations. The implementation, performance and limitations of the proposed WLI method are discussed in detail in the paper.
Lentil is a Python package for developing high-performance diffraction simulations. Lentil provides an easy to use framework for modeling optical systems and simulating the wave propagation of light through them. Traditional Fourier optics-based approaches for numerically modeling diffraction rely on the Fast Fourier Transform (FFT) for simulating free space propagation. Despite computational efficiencies provided by the FFT, these simulations can be slow and memory-intensive due to very large array sizes needed to satisfy numerical sampling requirements imposed by the FFT algorithm. Modeling large apertures, highly aberrated or misaligned systems, or small features like primary mirror segment gaps demand even finer sampling, further degrading performance. Directly computing the discrete Fourier transform (DFT) in diffraction calculations provides greater flexibility and increased performance when compared with computing an equivalent FFT. Lentil offers generalized diffraction propagation routines using the DFT that improve simulation performance substantially, with additional optimizations for modeling segmented apertures. Lentil also implements a hybrid propagation algorithm blending physical and geometric optics to greatly improve performance in simulations where representing large tilts is required. Additionally, Lentil includes tools for modeling static and dynamic wavefront errors, radiometry, and focal plane arrays. The Lentil package and its accompanying documentation are freely available as open-source software.
KEYWORDS: Adaptive optics, Wavefront sensors, Digital signal processing, Control systems, Signal processing, Imaging systems, Telescopes, Observatories, Coronagraphy, Detector development
PALM-3000 (P3K), the second-generation adaptive optics (AO) instrument for the 5.1 meter Hale telescope at Palomar Observatory, underwent a significant upgrade to its wavefront sensor (WFS) arm and real-time control (RTC) system in late 2019. Main features of this upgrade include an EMCCD WFS camera capable of 3.5 kHz framerates and advanced Digital Signal Processor (DSP) boards to replace the aging GPU based real-time control system. With this upgrade P3K is able to maintain a lock on natural guide stars fainter than mV=16. Here we present the design and on-sky re-commissioning results of the upgraded system.
The HabEx mission concept is intended to directly image planetary systems around nearby stars, and to perform a wide range of general astrophysics and solar system observations. The baseline HabEx design would use both a coronagraph and a starshade for exoplanet discovery and characterization. We describe a lower-cost alternative HabEx mission design, which would only use a starshade for exoplanet science. The starshade would provide excellent exoplanet science performance, but for a smaller number of detected exoplanets of all types, including exoEarth candidates, and a smaller fraction of exoplanets with measured orbits. The full suite of HabEx general astrophysics and solar-system science would be supported.
Future adaptive optics systems will require advanced predictive control algorithms that mitigate loop latency by forecasting disturbances. In many applications, such as laser communications terminals, synthesizing such controllers is challenging due to the non-stationary nature of the disturbance statistics over long periods. We present initial experimental results using a new, multichannel, adaptive control algorithm applied on the Integrated Optical System, an extreme AO system under development for the Laser Communications Relay Demonstration. The adaptive controller implicitly tracks disturbance statistics, and provides broadband wavefront control without the need for open loop downtime. The results illustrate the improved disturbance rejection capabilities of the controller compared to a traditional integrator, even when the exogenous disturbance statistics evolve.
The Integrated Optical System (IOS) is an extreme adaptive optics system designed for NASA’s Laser Com- munication Relay Demonstration mission. There is a great deal of overlap between the requirements for laser communication AO and high-contrast exoplanet imaging AO systems. Both require very high Strehl ratios with narrow fields of view. This overlap allows the IOS to serve as a testbed and technology demonstrator for astronomical extreme adaptive optics systems.
There are several example technologies from the IOS that are already making the transition to astronomical AO systems. The first is that the real time controller based on Direct Memory Access transfer between the WFS camera link frame-grabber and a DSP board is being reused on the upgrade to PALM-3000 AO system at Palomar Observatory. This enables the system to minimize latency by bypassing the CPU and its inherent timing jitter. Technologies like this will be crucial to enabling high contrast imaging on the next generation of extremely large telescopes. In addition, the IOS measures Fried’s parameter from wavefraont measures in near real time. This technology has already been deployed to PALM-3000. The main function of Laser Communication AO systems is to couple the incoming light into single mode fiber. This is the same configuration that will be used by AO coupled radial velocity spectrographs.
The adaptive optics system is a woofer/tweeter design, with one deformable mirror correcting for low spatial frequencies with large amplitude and a second deformable mirror correcting for high spatial frequencies with small amplitude. The system uses a Shack-Hartmann wavefront sensor. The system has achieved first light and is undergoing commissioning. We will present an overview of the system design and initial performance.
The HabEx mission concept is intended to directly image planetary systems around nearby stars, and to perform a wide range of general astrophysics and solar system observations. Its main goal is the discovery and characterization of Earthlike exoplanets through high-contrast imaging and spectroscopy. The baseline HabEx concept would use both a coronagraph and a starshade for exoplanet science. We describe an alternative, “HabEx Lite” concept, which would use a starshade (only) for exoplanet science. The benefit is lower cost: by deleting the complex coronagraph instrument; by lowering observatory mass; by relaxing tolerances and stability requirements; by permitting use of a compact on-axis telescope design; by use of a smaller launch vehicle. The scientific penalty of this lower cost option is a smaller number of detected exoplanets of all types, including exoEarth candidates, and a smaller fraction of exoplanets with measured orbits. Our approach uses a non-deployed segmented primary mirror, whose manufacture is within current capabilities.
The Palomar Ultraviolet Laser for the Study of Exoplanets (PULSE) will dramatically expand the science reach of PALM-3000, the facility high-contrast extreme adaptive optics system on the 5-meter Hale Telescope. By using an ultraviolet laser to measure the dominant high spatial and temporal order turbulence near the telescope aperture, one can increase the limiting natural guide star magnitude for exquisite correction from mV < 10 to mV < 16. Providing the highest near-infrared Strehl ratios from any large telescope laser adaptive optics system, PULSE uniquely enables spectroscopy of low-mass and more distant young exoplanet systems, essential to formulating a complete picture of exoplanet populations.
We report on the status of PALM-3000, the second generation adaptive optics instrument for the 5.1 meter Hale telescope at Palomar Observatory. PALM-3000 was released as a facility class instrument in October 2011, and has since been used on the Hale telescope a total of over 250 nights. In the past year, the PALM-3000 team introduced several instrument upgrades, including the release of the 32x32 pupil sampling mode which allows for correction on fainter guide stars, the upgrade of wavefront sensor relay optics, the diagnosis and repair of hardware problems, and the release of software improvements. We describe the performance of the PALM-3000 instrument as a result of these upgrades, and provide on-sky results. In the 32x32 pupil sampling mode (15.8 cm per subaperture), we have achieved K-band strehl ratios as high as 11% on a 14.4 mv star, and in the 64x64 pupil sampling mode (8.1 cm per subaperture), we have achieved K-band strehl ratios as high as 86% on stars brighter than 7th mv.
A state-space disturbance model and associated prediction filter for aero-optical wavefronts are described. The model is computed by system identification from a sequence of wavefronts measured in an airborne laboratory. Estimates of the statistics and flow velocity of the wavefront data are shown and can be computed from the matrices in the state-space model without returning to the original data. Numerical results compare velocity values and power spectra computed from the identified state-space model with those computed from the aero-optical data.
A new method for adaptive prediction and correction of wavefront errors in adaptive optics (AO) is introduced. The new method is based on receding-horizon control design and an adaptive lattice filter. Experimental results presented illustrate the capability of the new adaptive controller to predict and correct aero-optical wavefronts derived from recent flight-test data. The experimental results compare the performance of the new adaptive controller the performance of a minimum-variance adaptive controller previously used in AO. These results demonstrate the reduced sensitivity of the receding-horizon adaptive controller to high-frequency sensor noise.
This paper introduces a new method for adaptive prediction and correction of wavefront errors in adaptive
optics. The new method is based on receding-horizon control design and an adaptive lattice filter. Experimental
results presented in the paper illustrate the capability of the new adaptive controller to predict and correct
aero-optical wavefronts derived from recent flight-test data. The experimental results compare the performance
of the new adaptive controller the performance of a minimum-variance adaptive controller previously used in
adaptive optics. These results demonstrate the reduced sensitivity of the receding-horizon adaptive controller to
high-frequency sensor noise.
This paper compares an optimal linear time-invariant controller and an adaptive controller for prediction and
control of aero-optical wavefronts derived from recent flight-test data. Both control methods have the form of
multichannel prediction filters that capture the statistics of the aero-optical turbulence to mitigate latency in
the adaptive optics loop. Experimental results show the improvement in wavefront correction achieved by both
prediction methods. Altering the flow characteristics of the disturbance wavefront during the control process
illustrates the ability of the adaptive controller to track changes in the aberration statistics.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.